Patent GB-695199-A (GB)
Title: Improvements in and relating to electrical pulse code signalling systems
Inventor: LEVY MAURICE MOISE
IPC: H03M1/00
Filed: 1949-12-23
Abstract: 695,199. Multiplex pulse code signalling. GENERAL ELECTRIC CO., Ltd. Dec. 22, 1950 [Dec. 23, 1949], No. 32994/49. Class 40 (v)] Apparatus for producing a pulse code modulation signal in which the coded pulses are representative of a characteristic of a signal which is to be transmitted comprises means for periodically sampling the signal, coding means for producing a first impulse code train which represents the said characteristic to a first degree of accuracy, means to amplify the error signal corresponding to the difference in the characteristic of the original signal and the characteristic represented by the first impulse code train, and coding means for producing a second impulse code train representative of the amplified error signal, the duration of the first and second impulse code trains being equal to or less than the time between successive samples to be coded. The coding circuit is similar to that described in Specification 695,198, but the code pulse group comprises eight pulse positions providing 256 codes. for different amplitude levels. Five channels are provided. Sample pulses F are obtained in known manner, whose amplitude S represents the signal amplitude at predetermined intervals. The waveform F is applied to the control grid of a valve 19 having its cathode connected to the control grid of an integrator valve 64, and the voltage produced across the virtual condenser 65, which is equivalent to condenser 20, is equal to S, a like voltage appearing across the resistor 21 in the cathode circuit of valve 22. A waveform E is produced by shockexciting a tuned circuit by means of pulses D, the amplitude of each peak being one half that of the preceding peak, and is applied to the control grid of a valve 24, the voltage at 25 being the difference between the voltage across condenser 65 and the waveform E as amplified by valve 24, as shown between 47 and 40 curve G. The waveform G is applied to the control grid of a valve 26 and through a clamping circuit 29, to a gating circuit 36, 37, 43, controlled through a diode 44 by pulses A as described in Specification 695,223, so that when pulses B and -B are applied to circuit 29 the condenser 35 is charged to the potential of point 28, i.e. 39 on curve G, this potential appearing at point 38 to cause the diode 41 to conduct and the cathode of valve 19 to assume the potential of point 38. When pulses A are applied to the diode 44 the diode 41 is prevented from conducting, so that the cathode of valve 19 assumes the level 15 in curve G. After application of the second peak of curve E to the valve 24 the cathode of valve 19 assumes the level 16 in curve G. Point 25 is also connected to a diode 49 which conducts when the waveform G falls below the datum line as at 59 and provides a positive pulse at the anode of valve 50 to trigger a multivibrator 51, 52, and produce at 53 the leading edge 54 of waveform H, the time constant being arranged so that application of a pulse A to valve 52 generates the trailing edge 56. When a positive pulse of waveform H is applied to a diode 46 the diode 41 cannot conduct and the voltage at the cathode of valve 19 remains at its previous level, i.e. at point 17, curve G, which is the same as that at 16. Waveform H is also applied through a phase inverter valve 60 to the suppressor grid of a valve 61 to allow the pulses A applied at its control grid to appear at the output 63 only when the suppressor grid is positive and produce the first part of the pulse code representation I. After the fourth pulse in the train the resultant of the successive subtractions S<SP>1</SP> is multiplied sixteen times in amplitude as shown at 470 curve G and successive peaks of the next train of waveform E are then subtracted. The multiplication is carried nut by the circuit including valve 64 and condenser 20. The waveform K is applied to the grid of a valve 68 which is initially non- conducting but whose grid is driven positive shortly after the fourth pulse of waveform E, the valve 68 acting as a cathode follower to clamp the anode of valve 64 at a potential corresponding to that when the grid of valve 64 is at earth potential and valve 68 is cut off. This is a random occurrence since at the end of each coding cycle there may be a small level S<SP>1</SP> stored by the condenser 65. Valves 69, 70 and 71, limit the voltage lift of the anode of valve 64 has a value fifteen times that of the resistor 76 in the cathode circuit. After the fourth pulse of waveform E the anode voltage of valve 64 is a measure of the level S<SP>1</SP> but its anode potential is then clamped by the valve 68 and the operating conditions are such that a voltage of 1651 is developed at point 201. Pulses C are applied to the diode 45 and prevent the diode 41 from conducting whilst the voltage at 201 is increasing in magnitude. The code pulses may take other forms. Finite significance may be given to the condition where there is no pulse and zero significance when a pulse is transmitted, or alternatively pulses in one sense or the other may be transmitted in all the intervals. Instead of the waveform E a stepped waveform may be produced and compared with the sample level S.